In conventional concrete mixing, aggregates such as sand and stone are added together with cement powder and water, and then mixed to form concrete. Generally, the ingredients are mixed either in a central plant mixer or in a rotating truck mounted bowl mixer.
It has been found, however, that mixing times are reduced by up to 30% if a slurry of dry cement and water is mixed before the aggregate is added. In addition to the shorter mixing times, this method results in a more homogeneous mixture. Moreover, because the aggregate achieves a more uniform coating of cement slurry a higher strength concrete is produced compared to that produced by the conventional dry batch mixing method.
For these reasons, cement blenders have been developed for the purpose of initially mixing the dry cement powder with water to produce a cement slurry, which is subsequently discharged into a central or truck mixer for the addition of aggregrate.
Known cement blenders typically comprise a mixing chamber containing a mechanism for stirring the slurry, and a pump drawing from the bottom of the chamber to pump the slurry back into the top of the chamber until thoroughly mixed after which the same pump is used to deliver the slurry to the concrete mixer. However, blenders of this type are limited to mixing slurries of a cement to water ratio of less than about 2:1. If a higher ratio is attempted, for example 3:1, the slurry becomes too thick to interact with the stirring mechanism, which then rotates in a dead space within the thick slurry mixture. The slurry also sticks to the walls of the chamber and so cannot migrate to the outlet. This in turn causes the pump to cavitate, resulting firstly in possible wear and damage to the mixer, and secondly in the bulk of the slurry remaining in the mixing chamber unable to be mixed or discharged.
This problem can increase if other fillers are added to the cement particulate which is common practice. For example, silica fume is used to increase the strength of the final concrete mix and fly-ash is used as a cheaper substitute for cement particulate. It is quite common to have 80% cement particulate and 10% by weight each of silica fume and fly-ash. Because these fillers have a lower specific gravity than cement, their presence further increases the particulate to water ratio of the slurry.
Tests have shown that only about half of the water added is needed directly for hydration of the cement, whilst the remainder is required for reducing viscosity to facilitate pumping and to increase the workability of the final concrete. Unfortunately however, any increase in water content of the slurry or the concrete beyond the minimum requirement for hydration is inversely proportional to the strength of the concrete finally produced. Consequently, known cement blenders, being unable to mix and pump slurries having a cement to water ratio of more than around 2:1, are unable to produce concrete having optimum strength characteristics.